1. Technical Field
The present invention relates generally to communication networks utilizing spread spectrum radio transceivers, and, more specifically, to multi-hop RF networks wherein participating devices utilize spread spectrum transceivers that are capable of operating in any of a variety of spread spectrum modes. The spread spectrum modes include, for example, direct sequence transmission across a spreading bandwidth or channelized across the spreading bandwidth, frequency hopping transmission across all or a part of the spreading bandwidth, a hybrid combination of direct sequence transmissions and frequency hopping transmissions, and transmissions on a portion of the spreading bandwidth. The selection of a spread spectrum mode of operation depends upon signal conditions and characteristics of members capable of communication within the RF communication network.
2. Description of Related Art
Communication devices within a wireless local area network employ wireless communication links to transfer data and commands within the local area network. Typical units within a wireless local area network include stationary wireless access devices, mobile radio units, mobile image capture units, printing units, and other units operative with the data and commands. These units often link to a wired local area network through a wireless access device to transfer data and commands to devices located on the wired network. The wireless local area networks typically employ cellular communication techniques to provide the wireless communication links within the local area network.
One common installation of a wireless local area network serves factory automation functions. Because hard-wiring a local area network within a large, dynamic facility is both expensive and difficult, the wireless local are network provides traditional network functions as well as additional functions germane to the wireless attributes of the network. However, due to difficult transmission and interference conditions within a factory, establishing and maintaining sufficient wireless communication ties oftentimes proves difficult. Attenuation or transmitted signals, multi-path fading, ambient noise, and interference by adjacent cells often disrupts communication within the wireless local area network.
Spread spectrum transmissions are often used in attempts to overcome communication problems. With spread spectrum transmissions, the bandwidth over which information is broadcast is deliberately made wide relative to the information bandwidth of the source information. Spread spectrum transmission techniques include direct sequence transmission, frequency hopping transmission, a combination of direct sequence transmission and frequency hopping transmission, and may include other techniques that deliberately transmit over a wide spectrum.
Direct sequence spread spectrum transmitters typically spread by first modulating a data signal with a pseudo random chipping sequence at a multiple of the source data clocking rate. Once constructed, the composite modulation is coupled to a carrier via modulation techniques and then transmitted. Phase modulation is typically employed, but frequency modulation or other types of modulation may also be used. Circuitry in a receiving units receives the signal, decodes the signal at the multiple of the source data clocking rate using a particular chipping sequence, and produces received data. In a typical direct sequence system, the pseudo random chipping sequence applied by the receiving unit corresponds to all, or respective portions, of the transmitted signal. In this fashion, the receiving unit receives only intended data and avoids receiving data from adjacent cells operating on the same frequency. Direct sequence spread spectrum modes also provide significant noise rejection characteristics since each component of the source data is essentially transmitted multiple times. The received signal is therefore a composite that may be averaged or weighted to avoid receiving improper data or falsing based upon noise.
A frequency hopping system commonly uses conventional narrowband modulation but varies the modulation frequency over time in accordance with a known pattern or algorithm, effectively moving the modulated signal over the intended spreading bandwidth. The spread spectrum signal is only discernible to a receiver that has prior knowledge of the spreading function employed and which has obtained synchronization with the spreading operation at the transmitter. By spreading transmissions over the spreading bandwidth, particular portions of the spreading bandwidth within which transmission is difficult may be substantially avoided.
In the United States and many other countries, spread spectrum communications is used commercially within designated Industrial, Scientific and Medical (ISM) bands. These bands are structured as multi-use bands containing non-communications equipment such as industrial and commercial microwave ovens as well as low power consumer grade transmitters, vehicle location and telemetry systems and other spread spectrum devices of differing characteristics. Operation in ISM bands is unlicensed and uncoordinated, so equipment operating in these bands must be designed to operate successfully without knowledge of the types of devices that may be used in close proximity. The spread spectrum system design must also take into consideration the occupants of the spectrum adjacent to the ISM bands which may be both potential sources of interference to, and susceptible to interference from, various types of spread spectrum products.
Various forms of modulation across the spreading band may be utilized in commercial spread spectrum packet data communication systems. Full band direct sequence systems occupy the entire width of an ISM band. The spreading ratio, the ratio of the bandwidth of the spread spectrum modulated signal to the information bandwidth of the source modulation, determines the process gain of the system. Regulations within the United States mandate a minimum process gain of 10 dB, which is determined from ten times the logarithm of the spreading ratio. Process gain is a measure of the ability of a spread spectrum system to resist interference. The larger the spreading ratio, the more resistant the system is to interference within the receiver bandwidth. Wide bandwidth modulation is reasonably resistant to low or moderate levels of interference, but even systems with relatively high process gains experience difficulties when subject to strong interference.
When system throughout requirements dictate high data rates, the minimum process gain requirements in the regulations necessitate using wide bandwidth transmissions. For example, a well-known system NCR Wavelan uses Quadrature PSK modulation at 1 million symbols per second to achieve 2 megabits per second (MBPS) data rates with a source information bandwidth available in the US ISM band at 902 MHz band. In practice, implementation constraints dictate that this system uses the full 26 MHz band. Systems operating at other data rates, including the original Norand system, utilize the full bandwidth at lower source data rates, e.g., 200 kilobits per second (KBPS). Utilization of a wider spreading bandwidth in this case provides greater rejection of multipath fading typical of the indoor RF signal propagation environment.
When it is anticipated the direct sequence systems may be used in environments with strong in-band interference, a design choice is to employ channelization to reject interference. In the case of channelized direct sequence (DS) modulation, the spreading bandwidth is reduced to a fraction of the total available bandwidth, and a frequency-agile frequency generation systems is employed. By selecting the carrier frequency of operation, communications can be established in a portion of the band where interference is not present. This technique requires the use of selective filters in the receiver intermediate frequency (IF) section to provide the necessary interference rejection. These channelized DS systems utilize interference avoidance rather than relying on process gain to reject interference.
Utilization of frequency hopping spread spectrum systems is appropriate in environments where interference within the band of operation is not confined to particular portions of the band, but may periodically arise in various parts of the entire band. Frequently hopping is also useful as a multiple access technique. Use of multiple hopping sequences concurrently within a given location allows many simultaneous communication sessions to be supported. Occasionally, devices operating on different hopping sequences will simultaneously occupy the same channel within the band for short periods of time. For moderate numbers of simultaneous hopping sequences, this occurs infrequently.
Frequency hopping also provides similar multipath rejection capabilities to wideband direct sequence modulation. If a particular channel of operation is in a fade temporarily preventing communication, a jump to a frequency sufficiently removed from the faded frequency will often allow communications to resume.
Frequency hopping systems require more protocol overhead to aid in establishing and maintaining synchronization between units sharing a given hopping sequence. Additionally, the initial acquisition of the hopping sequence may require that an unsynchronized device scan the band for a period equivalent to may hop times. The overhead for direct sequence systems is lower, with several bit-times usually allocated to receiver acquisition at the beginning of each transmission.
Spread spectrum communications may not be appropriate for some applications. For example, short hop communications such as communications between a portable hand-held terminal and a peripheral device such as a scanner or printer over a short distance is a very cost sensitive application. Spread spectrum operation requires more circuit complexity and power consumption than is tolerable for this application. Simpler FM or AM techniques such as ON-OFF-Keying (OOK) may be desirable.
Conventionally, the particular spread spectrum modulation technique is chosen according to the particular applications in which the data transceiver is to be utilized. For example, in a small warehouse having few RF barriers, minimal interference from cellular and wireless phones, and minimal amounts of communication traffic, radio transceivers used therein might only employ direct sequence spread spectrum transmission techniques. Thus, conventionally, such transceivers would be specifically designed, constructed and installed. However, after installation, if communication traffic or local noise increases, the communication might fail to function as required. Likewise, after installation, if RF barriers are installed or if the network is moved to an urban environment with a great deal of noise from neighboring installations, cellular and mobile phones, etc., the network may fail to meet the needs of the customer.
Similarly, a design might be based on a customer's needs for a small store in a downtown urban area. Because of the greater likelihood of a great amount of radio frequency traffic in the vicinity, the customer requires a radio which is free from interference from nearby radio transmissions with little concern for operating range. Consequently, a different specific type of radio would be designed to meet the needs of the corporation based upon the operating conditions in which the radio is to be used, for example using frequency hopping modulation.
In the exemplary installations mentioned above, each of the radios would be optimized to meet the needs of the customer. However, a customer's needs continually change, and, if the particular application or environment were to change justifying a different spread spectrum modulation technique, the customer is either forced to change all of their radio transceivers or live with the under-performance they currently receive.
Moreover, in a typical network installation, a client may have diverse operational requirements. For example, the particular applications of the radio unit may change several times within the same day. The site may also have areas which are relatively noise and barrier free and those which encounter heavy noise and barriers. Some areas may have high traffic volume, while others experience only occasional traffic. In such networks, a single radio transceiver design can never provide optimal performance in all areas. Sacrifices are made in the design characteristics of the transceivers in an attempt to provide best performance overall.
Similarly, in mobile contexts, a worker may require mobile communications to a vehicle based information system or forwarding to a central communication facility through a vehicle based radio WAN transceiver. The characteristics of the communications medium for this class of operation vary greatly. Interference will vary from location to location. Additionally, it is necessary to allow operation if the worker moves away from the vehicle or inside a building structure. Because each wireless local area network may have been designed for a particular set of criteria with particular spread spectrum operational abilities, mobile units may be non-functional within particular wireless local area networks.
Thus, there is a need in the art for a communication network that operates dynamically to optimize communication utilizing various spread spectrum transmission techniques, considering the characteristics of RF noise, neighboring interference, RF barriers, participating transceiver unit capabilities and applications to be performed in such dynamic optimization.
It is another object of the present invention to provide a spread spectrum RF transceiver module, for use in wireless network devices, which utilizes multiple spread spectrum modulation techniques providing multiple configurable modes of data transmission, whereby modes may be selected to attain optimal transmission performance.
A further object of the present invention is to provide an RF data transceiver module which combines frequency hopping and direct sequence transmission techniques within a single design.
It is an object of the present invention to provide a spread spectrum RF transceiver module which utilizes common media access protocols and interfaces for multiple nominal carrier frequencies and modulation parameters.
It is a further object of the present invention to provide a spread spectrum RF transceiver utilizing 900 MHz transmission and having a standard interface with common 2.4 GHz transmission.
It is a further object of the invention to provide a spread spectrum RF transceiver which may be utilized in several different types of multi-layered data communications networks.
Another object of the present invention is to produce a wireless local area network and packet wireless data communication system that is flexible to operate reliably in varied and unpredictable RF propagation and interference environments.
A further object of the present invention is to provide a wireless RF transceiver module capable of utilizing a variety of operational modes thereby allowing large business operation to purchase a single product meeting a multiple usage needs maximizing operational flexibility and minimizing sparing and service concerns.
It is another object of the present invention to provide a modular wireless LAN modem capable of supporting multiple modes of operation under a single media access protocol with a standardized interface to a hand-held portable data terminal such that the wireless LAN modem may dynamically change modes of operation transparently to the host device, not requiring that the host device be aware of changes in the modes of operation, or that operation of higher protocol layers be impacted.
Yet another object of the present invention is to produce a modular wireless LAN modem that may be utilized for both in-premise and worker to vehicle application, and for short range communications to peripheral devices.
These and other objects of the invention will be apparent from examination of the drawings and remainder of the specification which follows.